What Do Oaks and the Irish Have in Common?

 This is a Quercus agrifolia, commonly known as a coast live oak, that was brought to my attention by a North San Gabriel Valley resident. The branches of the tree had defoliated over the last several years. The lawn is a year old, but the previous owner had grown ferns under the tree before that.

This tree is a great example of an oak that might be saved. Live oaks are notorious for defoliating under stress. You might wonder, considering the verdant and luxurious lawn that can be found under this tree, what the source of this defoliating stress is? Well consider the verdant and luxurious lawn for one...and the ferns the previous owner grew underneath the oak. Both of these require lots of water compete with the tree for nutrients. But simple competition isn't the only thing. Oomycetes (in particular Phytophtora species ) organisms that are commonly called water molds, are mostly to blame. Related to the organism that caused the Irish potato famine in the mid 19th century, these destructive organisms grow, multiply and thrive in watery environments. A well irrigated lawn is one such watery environment. The film of water that coats the soil particles in moist lawns is enough to allow the swimming spores of oomycetes to infect the roots the oak tree and cause considerable damage -as well as providing a gateway to a more destructive organism; oak root fungus. It's also a good idea to check the drainage of the area around your oak to make sure that any recent construction hasn't created an impairment for the runoff of water from the area.

So what can be done? The first thing is to tear out the lawn underneath the canopy (or the area that would have been the canopy if the tree wasn't defoliated) and mulch the area with shredded bark. Then allow the trees own leaves to build up in a thick layer underneath the tree. If recent construction has impaired water runof, fix it so that water drains away from the area where the oak is planted. Then treat the area with Agrifos according to instructions (Agrifos is a type of fertilizer that has a preventative action against oomycetes). And finally adjust your watering so that you are not watering the tree; natural rainfall is usually enough to sustain the tree. However, additional watering may be necessary in situations where it is a drought year and the original water gathering root area of the tree (which can be an area up to twice the diameter of the tree's healthy canopy) is compromised.

Here is a particulalry good publication that goes over what can go wrong with oak trees.